1. Field of the Invention
The present invention relates to a network system which an ATM (Asynchronous Transfer Mode) network is used as a relay for plural frame relay networks and to a frame relay switch comprised in those frame relay networks.
2. Description of the Related Art
Recently, the B-ISDN (Broadband Integrated Service Digital Network) is rapidly introduced as the LAN (Local Area Network) is made a high speed and a multimedia. Under this condition, in a data communication network, there are many cases that an ATM network is used as a network relaying plural frame relay networks. FIG. 23 is a schematic diagram illustrating a network which an ATM switch is used as a relay for plural frame relay networks. The network system is structured based on the ITU-T (International Telecommunication Union-Telecommunication Standardization Sector) recommendation Q. 922. Respective frame relay networks 1, 2 are connected with an ATM network 3 through IWFs (InterWork Function) 4. The frame relay networks 1, 2 are also connected with terminals 5, 6, respectively.
The IWF 4 is a converter converting a frame used for transmitting/receiving data in the frame relay networks 1, 2 to ATM cells used for transmitting/receiving data in the ATM network 3. When the IWF 4 receives a frame from the frame relay network 1 or the frame relay network 2, the IWF 4 divides (disassembles) the frame into plural cell data and adds cell headers thereto, respectively, so as to transmit to the ATM network 3. Moreover, when the IWF 4 receives plural ATM cells enough to disassemble one frame from the ATM network 3, the IWF 4 removes the cell headers from the plural ATM cells and assembles the respective cell data so as to assemble the frame, and then transmits the frame to the frame relay network 1 or the relay network 2 connected with the IWF 4.
In a network like this, when a congestion occurs in the frame relay networks 1, 2, the FECN (Forward Explicit Congestion Notification-bit) and the BECN (Backward Explicit Congestion Notification-bit) stored in a core header of the frame are set to "1", whereby it is recognized that the congestion occurs in the frame relay networks 1, 2. Moreover, when a congestion occurs in the ATM network 4, the congestion notification (PT: Payload Type (EFCI)) stored in a cell header of the ATM cell is set to "1".
However, the above-described conventional network system has the following problems. For instance, as shown in FIG. 24, when the terminal 5 transmits plural frames a-d to the terminal 6 as a transmitted side, a congestion occurs in the ATM network 3, and then the frames b, c are discarded in the ATM network 3, and the frames a, d are transmitted to the terminal 6. In this case, the frame relay network 2 can not recognize that the frames b, c are discarded in the ATM network 3 based on the procedure specified in the Q. 922 ANNEXA of the ITU-T recommendation.
Further, as shown in FIG. 25, in a case that the terminal 5 transmits plural frames a-d to the terminal 6 as a transmitted side, when a congestion occurs in the frame relay networks 1 or the ATM network 3, the FECN in the core header of the frame or the EFCI in the cell header is set to "1" and the congestion is informed to the frame relay network 2. Then, the IWF 4 at the side of the frame relay network 2 removes the cell header when the frame is assembled from the plural ATM cells. Thus, the content of the EFCI in the cell header or that of the FECN in the core header of the frame is set in a core header of a frame which is transmitted to the frame relay network 2. As the result, the frame relay network 2 can recognize that the congestion occurs in the frame relay network 1 or the ATM network 3 based on that the FECN is "1", but can not recognize in which of the frame relay network 1 or the ATM network 3.
As above described, there is a problem that the conventional network system can not recognize the quality of the ATM network suitably, therefore, when users of the terminals 5, 6 make complaints such as frames are often discarded or the throughput can not be raised caused by congestion notification frequent occurrence, administrators for the frame relay networks 1, 2 can not deal with these complaints suitably.